Blasting with particles such as sand or steel grit is in many respects a suitable method for cleaning or surface treatment. However, flying particles constitute an industrial health hazard, particularly when working with objects which cannot be completely enclosed in an air-tight box. It is, however, known that this disadvantage can be avoided by equipping the orifice, through which the blasting material is ejected, with a shield within which there exists an underpressure. This shield has an opening which is pressed against the object to be blast-treated and in this way the blasting material is prevented from spreading to the immediate surroundings and the treatment can be concentrated to a restricted area without the blasting material spreading diffusely over a larger area. By means of the underpressure the ejected sand is sucked away.
For blasting equipment of this kind it is known that safety features are incorporated which prevent the blasting material from being ejected if the shield opening is unrestricted and not pressed against a workpiece. Thus there is no risk of blasting material being ejected freely through the opening and thereby causing damage.
Such a known safety arrangement is based on registering the underpressure within the shield; if the shield opening is pressed against an object the underpressure is greater than when the opening is unrestricted.